The global terror from groups such as al-Qaeda represented a "new type of war" - and Mr Blair warned of his fears that they would obtain weapons of mass destruction.

The attacks of September 11th were a "revelation" to him, said Mr Blair, which had revealed the intentions of "fanatics".

"It is monstrously premature to think the threat has passed. The risk remains in the balance, here and abroad," he said.

The threat from terrorism was still present, warned Tony Blair

Fighting terrorism would mean relying increasingly on intelligence, said Mr Blair as he described the decisions that politicians had to confront.

"A short while ago, during the war, we received specific intelligence warning of a major attack on Heathrow. To this day, we don't know if it was correct and we foiled it or if it was wrong," said Mr Blair.

"But sit in my seat. Here is the intelligence. Here is the advice. Do you ignore it? But, of course intelligence is precisely that: intelligence. It is not hard fact. It has its limitations."

Terror's roots

Mr Blair also gave details of how intelligence had revealed a pattern of growing threats.

"We knew that al-Qaeda sought the capability to use weapons of mass destruction in their attacks. Bin Laden has called it a "duty" to obtain nuclear weapons," he said.

The true danger was not to any politician's reputation, but to our country

Tackling the threat of terror was not only about intelligence and military action, he said. It was also important to tackle the roots of extremism, he said urging more action on "poverty in Africa and justice in Palestine".

Mr Blair also called for an overhaul of the United Nations, so that its "security council represents 21st century reality". It must be able to "act effectively as well as debate".

'Physical evidence'

The prime minister has been seeking to re-focus on domestic issues, but he has been dogged by allegations about the legitimacy of going to war with Iraq.

Mr Blair said no other decision he had taken "had been so divisive" - and his speech represented his strongest defence so far of the decision making that led to war.

He asserted that it remained his "fervent view" that the danger posed by the Iraqi regime had been "real and existential".

"The true danger was not to any politician's reputation, but to our country," said Mr Blair.

He accepted there was so far no "physical evidence" of Iraqi weapons of mass destruction, he argued the threat of their development was indisputable.

UN worries

Conservative leader Michael Howard said during a visit to Scotland he would back any anti-terror action.

"I entirely agree with the prime minister that we face a real threat from terrorism and it is vital that we take effective action to deal with that fact."

"But there is another issue: competence. The prime minister should have asked whether the weapons of mass destruction he thought existed in Iraq were
battlefield weapons or weapons which could have been used against our troops in Cyprus.

"Any Prime Minister who takes us to war without asking fundamental questions
of that kind must expect to have his judgment questioned."

Liberal Democrat leader Charles Kennedy warned that the prime minister's calls for reforms of the United Nations seemed echo President George Bush's "do as I say" attitude.

"Unless we invest in the political credibility of the United Nations, we are going to be in very difficult long-term situations indeed," Mr Kennedy told BBC Radio 4's World at One.

Earlier, the decision to go to war against Iraq faced a further challenge from former chief UN weapons inspector Hans Blix, who said the conflict was illegal.